The present invention relates to a method and apparatus for identifying a material type in an article, such as a wholly or partly transparent bottle of plastic or glass, comprising, with the aid of a detector station, irradiating the article with rays from an infrared radiation source, detecting rays which have passed through the article non-absorbed, and then carrying out a correlation analysis of such detected rays, as is set forth in the preamble of the enclosed patent claims.
The invention also relates to a use of the method and apparatus in a reverse vending machine for identifying and sorting bottles of different types of material.
Identification of different material types, and especially types of plastics, is a problem area of growing importance, partly as a result of materials recycling becoming a progressively higher priority, from both a private financial and a socioeconomic point of view. If collection programmes for used materials are to be of maximum profitability, it is essential to ensure that the materials are as pure as possible as early as possible in the collection and handling chain. Pure materials have a value as raw materials for re-use, and there is a well-developed market and industry which handle such materials. If the materials are not pure, payment must often be made for their disposal.
A number of methods for identifying different types of plastics are previously known. A reliable method, which is frequently used, is spectroscopy in the infrared range. All known instruments for such spectroscopic identification of plastics are costly, as both the wave-length selecting elements (e.g., filters or gratings) and also the infrared sources and detectors are expensive.
As further illustration of the prior art in connection with, inter alia, the use of spectroscopy and infrared light, reference is made to U.S. Pat. Nos. 5,512,752, 4,719,351, 5,206,510, German patent publications DE 19601923, 19543134 and 4340795, and the Japanese patent applications JP-A-9138194, 6288913 and 6210632.
A number of other methods and equipment for the detection of plastics have also been developed which are somewhat cheaper than spectroscopic methods and equipment, but where the result of the detection is less reliable. Examples of such known equipment are the triboelectric detector and the optical double refraction detector.
Furthermore, the use of so-called correlation spectroscopy is known in connection with the measurement of gases, both for the detection of gases and concentration measurement. The gas to be analysed is used as a filter.
The object of the present invention is to use a similar technique for the detection of plastic materials. Absorption spectra of solid substances such as plastics are very different from the absorption spectra of gases. Whilst gases have very many, very fine lines in the spectrum, plastic materials have fewer and broader lines, so that as a rule the spectra of different materials have more or less overlapping lines. In such a situation, more information is obtained about the material to be identified by measuring the degree of spectral overlapping with a number of different plastic materials.
The method mentioned above is characterised, according to the invention, by
causing the article to pass through the detector station in a continuous or discontinuous movement;
causing the rays from the radiation source to be filtered successively by filters consisting of wholly or partly transparent materials having different spectral characteristics;
intercepting the rays filtered by the filter and non-absorbed by the article to form a sequence of measured values representing characteristic transmission signatures of the article; and
carrying out a correlation analysis of the signatures in relation to statistical models in order to determine the material type of the article.
Similarly, the apparatus mentioned above is characterised by
a conveyor controlled by a means to cause the article to move continuously or discontinuously and to pass through the detector station;
a device in the detector station provided with a plurality of filters consisting of wholly or partly transparent materials having mutually different spectral characteristics to effect successive and different filtration of the rays from the radiation source;
a means arranged to intercept the rays filtered by the filter and non-absorbed by the article and form a sequence of measured values representing characteristic transmission signatures; and
an analyser, e.g., a microprocessor, which is adapted to carry out a correlation analysis of the signatures in relation to statistical models in order to determine the material type of the article.
Here, statistical models should be understood to mean pre-established reference values, so-called calibration values which are related to signatures of certain types of material.
Additional embodiments of the method and the apparatus are set forth in the attached patent claims and in the description below with reference to the attached drawings.
As mentioned above, an advantageous application of the method and apparatus would be to use them in a reverse vending machine to identify and sort bottles of different material types.